An optical transmission system is designed to transmit and receive information by causing signals to propagate along an optical fiber transmission line. Since signals propagating along the optical fiber line attenuate (transmission loss), an optical fiber amplifier for amplifying signals is provided for a repeater to compensate for the loss. Conventionally, the C-band, defined as the wavelength range of 1,530 nm to 1,570 nm, is used as a signal wavelength band in the optical transmission system. As an optical fiber amplifier for amplifying signals in the respective channels in the C-band, an optical fiber amplifier (EDFA: Erbium-Doped Fiber Amplifier) having, as an amplification medium, an optical fiber for amplification (EDF: Erbium-Doped Fiber) with a core region doped with Er is used.
Recently, in order to realize larger-capacity information transmission, studies have been made to use signals in the respective channels in the L-band defined as the wavelength range of 1,570 nm to 1,610 nm in addition to the C-band. An EDFA can also be used as an optical fiber amplifier for amplifying signals in the respective channel sin the L-band. For example, as the first prior art, an Er-doped silica fiber and Er-doped fluoride fiber used as an L-band EDF are disclosed in Ono et al., “1.58-μm Band Er3+-Doped Optical Fiber Amplifier”, the 1997 IEICE General Conference, C-3-86 (1977).